1. Field of the Invention
This invention relates to a process for applying diffusants to solar cell semiconductor substrates using screen printing techniques.
2. Description of the Prior Art
Several semiconductor diffusant application methods are presently known. These include spinning, spraying, brush painting and vapor transport. (See, for example, U.S. Pat. No. 3,837,873.) However, each of these processes has significant disadvantages associated therewith.
In a typical spinning operation, one drop of a "diffusion source" solution is applied to the center of a semiconductor wafer, such as silicon, which is spinning at approximately 3,000 revolutions per minute. Centrifugal force spreads the material in a fairly uniform layer over the wafer's surface. However, non-uniformity due to perimeter pileup is a problem. Since the wafer is generally held down by a vacuum, there is a tendency for leakage. Some of the liquid dopant tends to flow off the edge and stain or contaminate the opposite side of the wafer. After the top surface film has been preliminarily dried but before diffusion is effected, a clean-up procedure is thus required. It is not usually feasible to coat both sides of a wafer with different dopants by this process. It also does not lend itself easily to automation.
Spraying is usually accomplished while the wafer is supported on a pedestal and is a form of spray painting. Once again, contamination of the opposite face may occur and both sides cannot be coated with different diffusants. It is also difficult to automate and non-uniformity is a problem.
In the brush painting technique, the dopant solution is applied by hand painting with a fine camel's hair brush. It is then typically dried for 15 minutes in air at 150.degree. C. The opposite face may be similarly hand painted with a different dopant and then dried in a similar manner before the wafer is diffused by heating at high temperature. The obvious disadvantages of this coating method, although it allows both sides to be coated and simultaneously diffused, are that the process is a hand operation, is necessarily slow and does not produce uniform or complete surface covering. Moreover, thickness variations in the dopant application may produce junction depth variations after diffusion. Of course, it is not automatable.
The vapor transport method is used to diffuse dopants into semiconductor wafers by having the dopant carried in a vapor from a source in a diffusion furnace. One method involves the placing of a solid source quite close to the wafer, e.g., Si, with a uniform spacing between the solid dopant source (often a similar wafer) and the surface of the silicon. When this process is used in an attempt to apply different diffusants to opposite faces of the wafer, e.g., P and B, cross-contamination occurs.
As can be seen, there is a need for an automatable technique of applying diffusants to one or both sides of a solar cell substrate in a reproducible and controlled fashion. With the current emphasis on the development of alternate energy sources, development of techniques to produce large quantities of solar cells at the lower costs possible with automation, is particularly critical. It would also be desirable to further production economy by simultaneously diffusing front and back dopants with a single diffusion firing. Such a process has been suggested (U.S. Pat. No. 3,895,975) but is subject to various disadvantages since it involves heating the substrate to a molten alloy state and also involves a pre-heating deposition of a diffusant by conventional techniques followed by a second vapor phase deposition during the actual heating step.